1. Field of the Invention
The present invention relates to an optical pickup device having an objective lens of a high NA for recording and reproducing information onto and from an optical recording medium, and an optical recording medium on which information is recorded by the device.
2. Description of the Background Art
Since light-employing techniques enable a large number of features such as high speed processing due to high frequencies, spatial information processing and phase processing, they are subject to research and development and put to practical use in wide-ranging areas including communication, measurement and processing.
For the techniques, high-precision objective lenses are used to reduce a light beam.
In recent years, expectations for image recording devices, particularly those employing light have been high, and techniques for mass storage capacity have become important. In addition to improving a recording medium, it is essential for mass storage of optical information recording to provide a smaller diameter of a beam spot, that is, to sufficiently narrow down a beam spot by using an objective lens.
As is well known, a beam spot diameter is proportional to a light wavelength and inversely proportional to an objective lens NA (Numerical Aperture). For the wavelength, blue laser diodes or blue or green SHG lasers have been developed in recent years. For the higher NA of an objective lens, higher density has been achieved such that the NA is 0.6 for DVDs (Digital Video Disks) as compared with 0.45 for CDs (Compact Disks). However, it has been difficult to attain much higher NAs with a single lens element with two aspheric surfaces. Therefore, an optical pickup device for attaining a higher NA by using a 2-group 2-element lens system is disclosed in Japanese Patent Laying-Open No. 10-123410.
FIG. 19 shows a 2-group 2-element objective lens 101 disclosed in Japanese Patent Laying-Open No. 10-123410. The objective lens has an NA of 0.85, and light which passes through objective lens 101 enters a transparent body 104 and forms a beam spot at an image point 105. Objective lens 101 is formed of a lens 102 on the light source side which has two aspheric surfaces and a second lens 103 on the disk side which is a plano-convex lens having a spherical shape for the convex surface.
Generally, in an objective lens of an optical pickup system, a laser light source has xe2x80x9cGaussian intensity distributionxe2x80x9d in which the intensity is the highest around the center of the optical axis of the objective lens and becomes smaller toward the rim of the objective lens. For a high NA objective lens, the power of the entire objective lens needs to be increased, which leads to the need for increasing the curvature of each surface. Since the angle of incident light is large at the rim of each surface of the objective lens, the transmittance of light is small and thus the amount of light is substantially reduced.
FIG. 10 illustrates relations between the NA of an objective lens and the transmittance through the outermost rim of the light emitting surface of the objective lens in which the refractive index of a lens 100b is 1.5. The transmittance through the light emitting surface substantially decreases with the NA of 0.7 or more.
Therefore, the light amount at the lens rim is reduced by influences of both the light amount distribution of a laser light source and the transmittance. As a result, the beam spot which is narrowed down by the objective lens is not made smaller enough, and a small diameter sufficient for a high NA is difficult to attain.
An object of the present invention is to provide an optical pickup device which includes an objective lens of a high NA capable of suppressing a reduction in the light amount at the rim of the lens.
In one aspect, an optical pickup device of the present invention for attaining the above object collects light emitted from a light source by an objective lens having a numerical aperture (NA) of 0.7 or more and irradiates an optical recording medium so as to record or reproduce information onto and from the optical recording medium. Under the condition, the objective lens and an antireflection film formed thereon are set so that the intensity of light incident on the outermost rim of the objective lens is 45% or more and 80% or less of the intensity of light, from the light source, incident on the center of the objective lens.
In another aspect, the light pickup device of the present invention has the same condition to the one aspect. Under the condition, the objective lens and an antireflection film formed thereon are set so that the transmittance through the outermost rim of the objective lens is 90% or more of the transmittance through the center of the objective lens.
According to the structure in each of the above aspects of the present invention, it is possible to prevent a reduction in the light intensity at the outer periphery of a beam spot which is an important element in determining a beam spot diameter and to provide a beam spot diameter sufficiently smaller for a high NA. It is therefore possible to realize superior jitter characteristics and higher density optical recording.
In one embodiment of the optical pickup device in each of the above aspects, the objective lens includes a 2-group lens element formed of two lenses, at least one of the two lenses has a single-layer antireflection film formed on a surface on the light source side, and the maximum incident angle to the surface is or is below a value calculated by the following expression:
xe2x88x924400xc3x97(NA)3+9549xc3x97(NA)2xe2x88x926917xc3x97(NA)+1730.
In another embodiment of the optical pickup device in each of the above aspects, the objective lens includes a 2-group lens element formed of two lenses, one of the two lenses that is on the light source side has a single-layer antireflection film formed on a surface on the optical recording medium side, and the maximum incident angle to the surface is or is below a value calculated by the following expression.
xe2x88x921067xc3x97(NA)3+2274xc3x97(NA)2xe2x88x921624xc3x97(NA)+422
In still another embodiment of the optical pickup device in each of the above aspects, the objective lens includes a 2-group lens element formed of two lenses, at least one of the two lenses has a multi-layer antireflection film formed on a surface on the light source side, and the maximum incident angle to the surface is or is below a value calculated by the following expression:
xe2x88x925707xc3x97(NA)3+12867xc3x97(NA)2xe2x88x929685xc3x97(NA)+2497.
In still another embodiment of the optical pickup device in each of the above aspects, the objective lens includes a 2-group lens element formed of two lenses, one of the two lenses that is on the light source side has a single-layer antireflection film formed on a surface on the optical recording medium side, and the maximum incident angle to the surface is or is below a value calculated by the following expression:
xe2x88x92600xc3x97(NA)3+1294xc3x97(NA)2xe2x88x92938xc3x97(NA)+264.
In the optical pickup device in each of the above aspects, the light incident on the objective lens is preferably elliptically polarized light or circularly polarized light having an ellipticity of 0.4 or more.
In still another aspect, the optical pickup device of the present invention has the same condition to the one aspect. Under the condition, at least one lens surface of the objective lens has an antireflection film designed so that a portion providing the maximum light transmittance is other than the lens center of the objective lens.
Even by such a structure, it is possible to prevent a reduction in the light intensity at the outer periphery of a beam spot which is an important element in determining a beam spot diameter and to provide a beam spot sufficiently smaller for a high NA. It is therefore possible to realize superior jitter characteristics and higher density optical recording.
An optical recording medium of the present invention collects light by an objective lens having an NA of 0.7 or more so as to record and reproduce information. An antireflection film is formed on a surface on the light incidence side, and the antireflection film is set so that the light incident angle providing the maximum transmittance is an angle other than 90xc2x0.
According to the optical recording medium, the transmittance and the specific transmittance are increased as compared with a case where an antireflection film is not formed. Therefore, the transmittance at the maximum light incident angle can be set optimally.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.